1. Field of the Invention
The present invention relates generally to a wireless communication apparatus such as a two-way radio, and particularly to a modulation-demodulation apparatus for use with a portable wireless communication apparatus such as a two-way radio.
2. Description of the Related Art
In the reception direct conversion technologies in which the receiving system of a suitable portable wireless communication apparatus such as a two-way radio uses a single local oscillator, when the transmitting system also uses only one local oscillator, it is customary that an oscillation frequency of a transmission local oscillator is selected to be twice as high as the transmission frequency (conversion coefficient A/B=1/2) and that an oscillation frequency of a reception local oscillator is selected to be twice as high as the reception frequency (conversion coefficient C/D=1/2). Specifically, in this case, the local oscillators of the transmission side and the reception side employ the same conversion coefficient ratio (frequency-dividing ratio). This direct conversion system has advantages such that its circuit system can be simplified and that its circuit scale can be made small.
When each of the transmission system and the reception system uses one local oscillator as described above, the two local oscillators have been mounted so far on separate modules or the two local oscillators have been incorporated within separate ICs (integrated circuits). Hence, there arises no problem. In recent years, in order to miniaturize device size in a portable wireless communication apparatus (two-way radio), such as a cellular phone, two local oscillators should be mounted within the same small module as close as possible or two local oscillators should be integrated within one chip. As a consequence, according to the above-described circuit system, a risk becomes remarkable, in which a fatal bad influence will be exerted upon reception sensitivity by interference between the two local oscillators.
To avoid such bad influence, there should be required some countermeasure for suppressing the occurrence of interference between substrates or the occurrence of interference in space. Accordingly, it is proposed to take a measure for covering assembly parts of the apparatus, for example, with shields or it is proposed to take a measure for locating the two local oscillators with a large physical distance therebetween. However, it is unavoidable that this measure, for example, causes a demerit from standpoints of freedom required when a substrate is designed, a design cost, a cost for shielding assemblies and mass-productivity.
Japanese laid-open patent application 2001-230695 describes an example of a wireless communication apparatus of direction conversion system in which a signal of the transmission system can be prevented from exerting a bad influence on the reception system. Basically, this direct conversion system technology essentially differs from the present invention in that it is assuming that one local oscillator is shared by the transmission system and the reception system. As a problem that arises when the transmission system and the reception system share one local oscillator, in the simultaneous transmission and reception system (e.g. CDMA (code division multiple access) system such as W-CDMA (wideband-CDMA) and cdmaOne or TDMA (time division multiple access) system such as a packet correspondence PDC (personal digital cellular)), when a signal is received at an intense electric field, there is a risk that the received signal will be leaked to the local oscillator. This leakage of the received signal deteriorates purity of signal and exerts a bad influence upon characteristics (e.g. accuracy of modulation, adjacent channel leakage electric power, spurious response, etc.) of the transmission system. Conversely, when a large signal is transmitted, there is a risk that the transmitted signal will be leaked to the local oscillator. This leakage of the transmitted signal exerts a bad influence upon characteristics (e.g. reception sensitivity, adjacent channel selectivity, spurious response, etc.) of the reception system. To avoid these shortcomings, it is necessary to take some countermeasure for decreasing leakages of the transmitted signal and the received signal into the local oscillator. For example, it is proposed that a buffer circuit may be added to the circuit arrangement of the portable wireless communication apparatus, which however, brings about a demerit from standpoints of a circuit scale, a cost and a time period necessary for awaiting incoming calls (i.e. time period necessary for making calls).